The present invention relates to the field of driving traction control and more particularly to a method for measurement of loss of traction between a drive wheel and a surface.
Traction of a vehicle is established through the constant and reliable contact of a wheel against a surface so that the wheel, when rotated by means of a driving force, will propel the vehicle along the surface in a selected direction. In the context of the present invention, traction refers to driving traction, not to braking traction. Traction results from the combination of the coefficient of friction and the force of the wheel against the surface. The force, or weight, of the wheel is directed vertically downward and may be analytically split into a proportional force normal to the surface and a proportional force parallel to the surface. The force normal to the surfaces is a part of the traction frictional force. The drive force is applied from the wheel to the surface along the tangent at the contact point of the wheel and the surface. A drive force, known as torque, that is great enough will overcome the frictional grip between the wheel and the surface and cause the wheel to slip in relation to the surface so that the wheel is rotating at a higher surface speed than its forward travel velocity. Once the frictional grip, or static friction, between the wheel and the surface is overcome by excess torque relative to the coefficient of friction, sliding or kinetic friction occurs. Kinetic friction is always less than static friction, providing a greater opportunity for slippage of the wheel and ultimately the vehicle. If the drive force that was sufficient to overcome static friction continues to be applied to the wheel, and the rotational speed of the wheel is restrained only by kinetic friction, the wheel will continue to rotate at a faster and faster speed.
The wheel slippage situation described above occurs frequently in driving an automobile. The slippage can occur due to excessive accelerative force applied to the wheels or due to inadequate wheel-to-road friction, e.g., due to inclement weather. Once a slippage condition begins, the driver has less control of the speed or direction of travel of the automobile. Automobile drivers typically attempt to control this slippage by reducing the speed at which the driving wheel is rotating, either by reducing the engine power through gasoline control or by applying the car""s brakes. Of course, the driver must first recognize the fact that slippage is occurring, then decide what is the proper correction to be made, then make the correction; all of this mental processing and reacting takes time. When driving in such conditions, the amount of time for a driver to evaluate and act on a situation is limited. If slippage occurs, for example, to a race car driver driving on a track and commonly traveling at a speed in excess of 240 kilometers (150 miles) an hour, the time available is very small and the ability to act correctly within that time is critical.
Numerous inventions have been directed to automatic control of slippage of a wheel against a road surface. In the prior art, automotive traction control systems disclosed in U.S. Pat. No. 5,519,617 to Hughes et al and U.S. Pat. No. 5,548,513 to Masuda et al are typical of those that employ a comparison of the rotational speed of the driven wheels to the rotation speed of the idler wheels to determine wheel slippage is occurring. According to these prior inventions, the maker of the vehicle must provide a speed sensor for each wheel whose speed is to be measured. Providing a wheel speed sensor at each of four wheels involves an added expense to the automobile. U.S. Pat. No. 4,615,410 to Hosaka discloses a system by which a target vehicle speed is determined by the vehicle gear selected, accelerator pedal position, and brake position, whereupon the target vehicle is compared to the rotational speed of a wheel of the vehicle to determine whether slippage is occurring. The system of the Hosaka patent is complex and potentially unreliable. A particular drawback of the known systems that compare the speed of a driven wheel with the speed of an idler wheel is that with a four wheel-drive vehicle, all wheels rotate at substantially the same speed at all times, and this system is inoperative.
The engine of most modern automobiles are equipped with a speed sensor that is adapted to provide information to an engine management system which regulates fuel, air, ignition timing, and other such operating factors. A system for determining wheel slippage, thus the need for traction control, that would utilize the existing engine speed sensor would avoid the need for the maker or owner of an automobile to add wheel-mounted speed sensors. Since the modern automobile engine management system is controlled by a microprocessor, and the existing engine speed sensor sends speed indicative signals to the microprocessor, modification of the engine management program to include traction control factors would accomplish this important goal reliably with a minimum of added cost. While a microprocessor is the preferred embodiment of the invention, it is recognized that similar results could be achieved by other devices capable of comparing two factors against each other. Traction control requires reducing the rotational speed of the drive wheels to match their travel over the road. Wheel rotational speed can be reduced by such factors as braking, fuel reduction, and ignition timing, amongst others. The choice of one or more of these control factors may be made on the basis of the degree of wheel acceleration that is occurring and road conditions.
Therefore, it is an object of the present invention to provide a traction control system for an automobile that determines wheel slippage and generates an automatic corrective action therefor.
It is an additional object of the present invention to provide a traction control system for an automobile that determines wheel slippage by means of an existing engine speed sensor.
It is a further object of the present invention to provide a traction control system for an automobile that generates an automatic corrective action in proportion to the degree of acceleration of the wheel.
These and other objects of the invention will become apparent from the disclosure to follow.
One form of the present invention provides a method for improving the traction control of an automobile that utilizes an existing engine speed sensor to determine the occurrence and degree of wheel slippage and generates an automatic corrective action to the engine, the vehicle""s braking system, or both, that is proportional to the degree by which the wheel is slipping. The traction control method of the present invention uses a computer program to evaluate a slippage situation and determine the amount of correction needed. The method provides for a series of measurements of the speed of engine or transmission rotation as determined by the existing sensors, comparing a first speed measurement to a second speed measurement, determining their difference, and comparing a selected maximum acceleration. In one embodiment of the invention, the selected maximum acceleration is manually set to a preset threshold. In another embodiment of the invention, the selected maximum acceleration is determined by constantly updating the average rate of acceleration of the engine. If the difference in speed measurements exceeds the limit, slippage is occurring. The degree to which the speed difference exceeds the selected limit determines the extent of needed corrective action.